The present invention relates to a method of preparing an inkjet printhead, prior to generating an image onto a receiving medium by the application of the printhead. The printhead contains multiple, substantially closed ink ducts, each having an inlet opening and a nozzle, said ducts each being operationally connected to a corresponding electro-mechanical transducer.
In order to prepare a printhead of this kind, which is known from the prior art, the ducts are often flushed with new ink. The ink is introduced into the duct via the inlet opening of the ducts, and flushed out via the nozzles, thus removing any contaminations, air bubbles, old ink residues and any other undesirable obstructions present in the ducts. In order to be virtually certain that all undesirable obstructions have been removed so that the printhead is ready to proceed and print an image, a relatively large amount of ink is flushed through the print head.
A disadvantage of this known method is that a relatively large amount of ink is lost when flushing the ink ducts of the print head. Furthermore, there is no absolute certainty that all undesirable obstructions (i.e. any disadvantageous state which has an adverse effect on the printing process, e.g. a dirt particle, an air bubble, a contaminated nozzle, an uneven ink temperature, incorrect ink, etc.) that may possibly be present in the ducts are actually removed by flushing the ducts. The present invention is directed to a method that obviates the above problems.
To this end, a method has been developed, wherein for each of the multiple ducts, it is arranged that the duct is filled with ink, a pressure wave is generated in the ink, the pressure wave causing a deformation of the corresponding transducer which generates an electrical signal as a result, the electrical signal is analyzed, and then it is determined whether the inkjet printhead is ready to proceed and print an image using an analysis of the electrical signal.
In this initial preparation, it is first guaranteed that the duct is filled with ink. If the initial process takes place with a printhead in which no “old” ink is present in the ducts, each duct must then first be filled with ink. If there is ink already present in the ducts, the filling process may be skipped if the presence of ink establishes that the ducts are already filled with ink. It is also possible that the ducts are partially filled with ink. In this case, only a partial refilling with new ink is required. The present method now comprises, for each of the said ink ducts, generating a pressure wave in the ink present in the duct. According to one embodiment, this pressure wave is generated by the actuation of a piezo-electrical transducer that is operationally connected to the duct. This may, for example, be the same transducer as the one referred to above. The pressure wave, in turn, causes a deformation of the transducer, which then generates an electrical signal as a result. As the form of the pressure wave depends on the conditions in the duct (the presence of air bubbles or dirt particles, for example, leads to the occurrence of another pressure wave), the electrical signal also depends on the conditions in the duct. Thus, by analyzing this signal, information on the conditions in the duct may be obtained. Based on this, it is possible to decide whether the duct is ready for jet ink printing. This information can be used to decide whether the print head as a whole is ready to print an image on a receiving material.
In the present method, it is no longer required to flush a relatively large amount of ink through each duct at each initial preparation. At the start of the initial process, i.e. without any ink having been flushed through the filled ducts, it is determined by application of the present method whether the ducts are ready. If it is determined, for example, that there are no undesirable obstructions present in the ducts, then the ducts are considered ready to transfer ink drops image-wise onto a receiving medium. In this case, it is therefore not required to flush new ink through the ducts. Furthermore, by application of the current method, there is a greater certainty regarding the actual readiness of the printer, as the state in the ducts is measured, whereas up to now it was customary to assume that the state was good after a large amount of new ink had been flushed through the duct. According to one embodiment, a repair action is applied if the printhead is not ready, after which the generation of the pressure wave, the resulting deformation of the transducer and the analysis of the signal generated by the transducer are repeated. According to this embodiment, for example, a small amount of ink is flushed through the obstructed duct in order to remove any undesirable obstruction which might be present. Another possible repair action, which is, for example, suitable for eliminating small air bubbles, is to temporarily leave the duct untreated to allow the air bubble to dissolve in the ink. Other repair actions, preferably geared to specific undesirable obstructions, are also possible. Once the repair action has been carried out, the state in the duct is again measured in the manner indicated above. If the undesirable obstruction has been removed by the repair action, it may be decided that the duct, and as a result possibly also the printhead, is ready to print. In this manner, a long initial preparation process may be avoided. As soon as the duct is free from undesirable obstructions, it may be decided that the printhead is ready to print. If the repair action that is chosen consists of flushing the duct with a small amount of ink, then the advantage of the current method would be that only a small amount of ink, that is an amount sufficiently adequate to remove the undesirable obstruction, is required to prepare the duct.
Moreover, it is known from European patent application EP 1 013 453 that an electro-mechanical transducer of an inkjet printer, apart from generating a pressure wave in the duct, may also be used as a sensor to obtain information on the state of the duct. However, from said application, it is only known to apply this in order to trace undesirable obstructions that occur during the printing process. It is not known from the European application to apply its method in order to check the duct for the presence of undesirable obstructions during the initial process, nor to decide, based on the European application thereof, whether the printhead is ready to proceed and print an image.
According to one embodiment, a pressure wave is generated such that an ink drop is ejected from a nozzle if the printhead operates normally. According to this embodiment, the generation of the pressure wave coincides with the ejection of an ink drop. The advantage of this embodiment is that the state of the ducts is measured under conditions that may be equivalent to that which exists during the actual use of the ducts during the printing process of an image. Furthermore, the additional advantage occurs that by jetting an ink drop, a small amount of ink is, in fact, flushed through the duct. In this manner, it is, for example, possible that no additional repair action is required to remove undesirable obstructions.
According to another embodiment, where the inkjet printhead comprises a collection of individually actuatable ink ducts and associated electro-mechanical transducers, the method comprises: preparing the printhead; deforming the associated transducer for each of the ducts of the collection; and analyzing the signals generated as a result. According to this embodiment, the method according to the present invention is applied to each duct of the printhead. Therefore, the preparation process takes place while each of the ducts are measured so that it may be determined for each of these ducts whether one or more undesirable obstructions are present. This information may be applied when deciding whether the printhead is ready to print an image.
In another embodiment, it is decided that the inkjet printhead is not ready if an undesirable obstruction is present in a duct. According to this embodiment, it is opted to only complete the preparation process of the printhead once each of the ducts is fully deployable to proceed to print an image. The advantage of this method is that it allows for optimal use to be made of the printhead and that it is not necessary to take ducts into account for printing which have a deviant property or absolutely lack the ability to jet ink drops for creating an image.
According to an alternative embodiment, it is decided that the inkjet printhead is ready despite an undesirable obstruction being present in an ink duct. According to this embodiment, it is opted to complete the preparation process of the printhead despite an undesirable obstruction being present in one or more ink ducts. The advantage of this embodiment is, for example, that the initial process is not required to be continued for a long time, unnecessarily, if there is an undesirable obstruction present in a duct that will not be required for printing the next image. In this case, the initial process may simply be completed, after which, the printing process of the image may be started. It may also be a fact that there is an undesirable obstruction present in a duct, the undesirable obstruction being of such a nature that it will disappear very quickly during the printing process and thus seldom produce any visible print artefacts. In this case, the present method allows the initial process to be completed despite the presence of an undesirable obstruction in one of the ducts.
According to another embodiment, the decision to print is made if there is at least a predetermined number of ink ducts without an undesirable obstruction. According to this embodiment, it is assumed that there is a minimum number of ink ducts required in the printhead for the printhead to be able to print an image. As soon as it appears from the initial process that this minimum number of ink ducts has been achieved, it may be decided that the printhead is ready to print an image.
According to another embodiment, the decision is made if it is determined that the undesirable obstruction is persistent. A persistent undesirable obstruction is one which cannot be removed, at least within a reasonable time, during the initial process. It may then still be decided that the printhead is ready nonetheless, where the duct in which the persistent undesirable obstruction is located will, for example, not be used during the printing process of the image. It may then be decided again at a later stage, for example, after expiration of or during the printing process of the image, whether the undesirable obstruction is still present or not.
According to one embodiment, the image is printed by application of those ducts which are free from any undesirable obstruction. This has the advantage that no print artefact need to occur in the image.
The present invention thus relates to an inkjet printer containing a printhead with a substantially closed ink duct containing an inlet opening and a nozzle, said duct being operationally connected to an electro-mechanical transducer, the printer including a control which has been modified such that it may control the printer to automatically carry out the present method.